The Influenza A virus, which belongs to the Orthomyxoviridae family, can cause influenza in humans, birds or domesticated food animals. The virus can be classified into different subtypes based on their surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). Of the 16 known HAs and nine NAs, three HA subtypes (H1, H2, and H3) and two NA subtypes (N1 and N2) are most commonly found in humans. H1N1 and H3N2 are the major subtypes that cause human seasonal flu and global pandemics of influenza. The influenza pandemic in 2009 is caused by influenza A virus H1N1 of swine origin. This has led to a growing concern regarding the pandemic potential of the highly pathogenic avian influenza (HPAI) H5N1 viruses. Thus the development of an effective and safe vaccine against divergent influenza A virus strains is urgently needed for the prevention of future outbreaks of influenza.